The present invention relates to an OFDMA (Orthogonal Frequency Division Multiple Access) signal transmission apparatus and its method.
In mobile communications, overcoming multipath fading and improving transmission quality have been conventionally present as problems to be solved. In connection with the multipath fading, there is a ii multi-carrier transmission as one of measures to transmit data with a good quality without lowering a symbol rate.
OFDM (Orthogonal Frequency Division Multiplexing) is one of multi-carrier transmission systems, that is, one that sets a distance between adjacent sub-carriers to 1/symbol rate to allow the distance between the sub-carriers to be narrowed at the maximum.
Also, OFDMA is a system in which a plurality of users performs multiple access using OFDM. In the conventional OFDMA data transmission apparatus, there is proposed a method in which frequency division and time division are carried out when multiple access is performed. Also, it has been considered that diversity is carried out in a frequency direction and a time direction to make it possible to improve an error correction capability.
FIG. 1 is a block diagram showing the configuration of a transmission section in the conventional OFDMA signal transmission apparatus. FIG. 2 is a block diagram showing the configuration of a reception section in the conventional OFDMA signal transmission apparatus.
A transmission section 10 shown in FIG. 1 mainly comprises S/P (Serial/Parallel) converter 11, IFFT (Inverse Fast Fourier Transform) apparatus 12, P/S (Parallel/Serial) converter 13, a D/A (Digital/Analog) converter and orthogonal modulator 14, a transmission amplifier 15, and a transmission antenna 16.
A reception section 60 shown in FIG. 2 mainly comprises reception antenna 61, quasi-coherent detector and A/D (Analog/Digital) converter 62, S/P converter 63, FFT (Fast Fourier Transform) apparatus 64, and P/S converter 65.
An explanation will be given of the operation of the conventional OFDM transmission and reception in the OFDMA signal transmission apparatus having the above-configured transmission section 10 and reception section 60. In this case, it is assumed that transmission section 10 and reception section 60 are provided in the base station of the mobile communication system (not shown) and the mobile station.
First, an explanation will be given of the operation, which is performed when forward signals are transmitted to the mobile station from the base station. In the case of the forward OFDMA, the base station performs the same operation as the case in which the sub-carriers are present in all bands of OFDM no matter how the sub-carrier is assigned to each mobile station.
Transmission data shown in FIG. 1 is data with respect to each mobile station, and transmission data is input to S/P converter 11 simultaneously, and converted in parallel. If the number of sub-carriers is N, N complex number values are set after conversion.
Here, 0 is placed at extra sub-carriers. The result is changed to time-waveform by performing Fourier transform of sub-carriers N by IFFT apparatus 12. Then, time series of sub-carriers N are arranged in order of time by P/S converter 13. Then, the result is converted to analog waveform, and orthogonally modulated by D/A converter and orthogonal modulator 14, thereafter converting the analog waveform to high frequency. Then, the resultant signals are amplified by transmission amplifier 15, and the amplified signals are emitted from transmission antenna 16.
On the other hand, in the mobile station, the emitted signals are received by the reception antenna 61 shown in FIG. 2 and the received signals are demodulated by quasi-coherent detector and A/D converter 62, thereafter converting the demodulated signals to digital values by S/P converter 63. Then, the converted signals are converted to parallel signals every N sample, thereafter the parallel signals are Fourier transformed to signals on a frequency axis by FFT apparatus 64. Moreover, the converted signals are converted to serial signals by P/S converter 65 so as to obtain received data.
Thus, since all sub-carriers are completely orthogonal to the forward signals, signals can be transmitted in a state that interference between the respective signals little occurs.
Next, an explanation will be given of the operation, which is performed when reverse signals are transmitted to the base station from the mobile station. In the case of the reverse OFDMA, as a method of multiple access, there are a method in which the sub-carriers are divided and a method in which time division is carried out using sub-carriers, or a method, which mixes the above two methods.
In the case of the method in which time division is carried out using all sub-carriers, the same operation as that of the case of forward signals is used, and only ON/OFF of transmission is added thereto.
The method in which the sub-carriers are divided will be explained. A certain mobile station is assumed. The mobile station converts transmission data to parallel data using S/P converter 11. If the number of sub-carriers assigned to the mobile station is N, N complex number values are set after conversion.
Data is changed to time-waveform by performing Fourier transform of sub-carriers N by IFFT apparatus 12. Then, time series of sub-carriers N are arranged in order of time by P/S converter 13. Then, data is converted to analog waveform, and orthogonally modulated by D/A converter and orthogonal modulator 14, thereafter converting the analog waveform to high frequency. Then, the resultant signals are amplified by transmission amplifier 15, and the amplified signals are emitted from transmission antenna 16.
The base station receives signals each having a different sub-carrier from a plurality of mobile stations. Synthetic signals of signals emitted from the plurality of mobile stations are received through the reception antenna 61, and the received signals are demodulated by quasi-coherent detector and A/D converter 62, thereafter converting the demodulated signals to digital values. Then, the converted signals are converted to parallel signals every N sample by S/P converter 63, thereafter the parallel signals are Fourier transformed to signals on a frequency axis by FFT apparatus 64. Moreover, the converted signals are converted to serial signals by P/S converter 65 so as to obtain received data.
In the reverse signals, if frequency offset is present every mobile station and maximum Doppler frequency is different, all sub-carriers are not completely orthogonal to the reverse signals. However, if these influences, which are exerted upon the symbol rate, are small, signals can be transmitted in a state that interference between the respective signals little occurs.
However, in the conventional OFDMA signal transmission apparatus, the following problems are present.
First, in the case of receiving the forward signals, since the sub-carriers can be separated only after FFT has been carried out, not only the sub-carriers assigned to the station but also the sub-carriers of all OFDM bands must be demodulated in the mobile station. For this reason, even if a traffic volume is low, the number of A/D converters and that of FFT apparatuses, which are the same as that of sub-carriers, must be provided. This increases in the circuit scale and power consumption.
Also, since the signals of all OFDM users (all mobile stations) are synthesized and the synthesized signals are amplified by the transmission amplifier in the base station, the dynamic range is large and it is difficult to restrain nonlinear distortion.
Moreover, since AFC (Automatic Frequency Control) is provided to only all OFDMA bands in the base station, deterioration in the quality of signals increases when the frequency offset differs depending on each mobile station or the maximum Doppler frequency is large.
A first object of the present invention is to provide an OFDMA signal transmission apparatus, which can reduce the amount of operations and power consumption, and its method.
This object can be achieved by converting a plurality of serial signals to parallel signals, arranging the plurality of converted serial signals at intervals of a power of 2 to perform sub-carrier assignment, and performing inverse Fourier transform to the number of sub-carriers, which has been varied depending on the number of rearranged parallel signals, to be transformed to time waveforms in the transmission section, which performs an OFDMA signal transmission.
A second object of the present invention is to provide an OFDMA signal transmission apparatus, which can restrain nonlinear distortion even if a transmission amplifier with a small dynamic range is used in a base station, and its method.
This object can be achieved by converting serial signals of the respective sequences to parallel signals, performing inverse Fourier transform with respect to the number of sub-carriers, which has been varied depending on the number of rearranged parallel signals, to be transformed to time waveforms, and converting the parallel signals to serial signals in the transmission section, which performs an OFDMA signal transmission.
A third object of the present invention is that high quality reception can be carried out even if frequency offset is different depending on each mobile station and maximum Doppler frequency is large in the base station.
This object can be achieved by making the frequency bands of the signals transmitted from the respective mobile stations different from each other, dividing received signals converted to digital signals for each frequency band, converting the signals to base band signals for the respective bands, limiting the signals, converting the limited signals to parallel signals, performing Fourier transform with respect to the signals, and converting the parallel signals to serial signals.